1. Field of the Invention
This invention generally relates to shaft seals, and is specifically concerned with a hydrostatic, film-riding seal that is useful for sealing the shaft of a pump used for pumping liquids at high pressures and temperatures such as a coolant water pump in nuclear power plant.
2. Description of the Prior Art
Coolant water pumps are used in commercial pressurized water nuclear reactors to continuously recirculate 100,000 gpm or more of coolant water through reactor coolant systems at temperatures of up to about 570° F. or higher and pressures of about 2250 psi or higher. A coolant water pump maintains the reactor coolant pressure and restricts leakage of coolant water along a rotating pump shaft with a series of shaft seals, including a primary seal assembly which is a hydrostatic, “film-riding” seal and secondary and tertiary friction (or “contacting” or “rubbing”) type seal assemblies. Most of the 2250 psi pressure differential between the reactor coolant system pressure and the surrounding atmosphere is dropped across the primary seal assembly. The actual seal is formed by two confronting faceplates, one of which is stationary (known as the “ring”) and the other of which turns with the pump shaft (known as the “runner”). Water is forced between the faceplates, which causes the faceplates to ride on a thin film of water that may be on the order of a half mil thick.
The correct flow of water between the faceplates of the primary seal (known as the “leak-off rate”) must be maintained within specifications for proper functioning of the seal. Investigations have shown that out-of-specification leak-off rates are often caused by iron oxide deposits on one or both faceplates. The predominant iron oxide phase deposited on the faceplates was found to be α-Fe2O3,i.e., the mineral hematite. In addition, lesser quantities of α-FeOOH, the mineral goethite, have also been detected. The iron in both hematite and goethite is in the +3 valence state.